1. Field of the Invention
The present invention relates to a process for preparing methacrylic acid from isobutane by    A) subjecting the isobutane in a reaction zone A to a partial selective heterogeneously catalyzed dehydrogenation in the gas phase to form a product mixture A which comprises isobutene and unconverted isobutane,    B) using the isobutane- and isobutene-containing product gas mixture A to charge a reaction zone B and subjecting the isobutene in reaction zone B to a selective heterogeneously catalyzed partial oxidation in the gas phase using molecular oxygen to form a methacrolein-containing product gas mixture B with the proviso that the molar conversion of isobutene is ≧95 mol % and    C) using the methacrolein-containing product gas mixture B without preceding removal of components contained therein to charge a reaction zone C and subjecting the methacrolein in reaction zone C to a selective heterogeneously catalyzed partial oxidation using molecular oxygen in the gas phase to form a methacrylic acid-containing product gas mixture C.
2. Discussion of the Background
Methacrylic acid is an important staple chemical which is used as such and/or in the form of its methyl ester for preparing polymers which are used, for example, finely dispersed in an aqueous medium as a binder.
DE-A 3313573 discloses a process for preparing methacrylic acid from isobutane as described at the outset. A procedure considered particularly advantageous by DE-A 3313573 involves using the product gas mixture A without preceding removal of components contained therein for charging reaction zone B and using pure oxygen as the source for the molecular oxygen required in reaction zone B.
However, disadvantages of such a procedure are that on the one hand the selectivity of methacrylic acid formation per se and on the other hand the extent of formation of the by-produced isobutyraldehyde and isobutyric acid, which are particularly undesirable even in very small quantities since they are difficult to remove from the target product and are particularly troublesome in subsequent uses, is not completely satisfactory.